This disclosure is directed to a method of descaling stainless steel using a descaling apparatus as disclosed in U.S. Pat. No. 7,601,226, the disclosure of which is incorporated by reference herein. In particular, the disclosure is directed to using such a descaler in a processing line having other machinery configured for pushing or pulling the stainless steel strip through the line.
Generally speaking, hot rolled carbon steel HRCS may be processed in a processing line having machinery configured for pushing or pulling the hot rolled carbon steel strip through the line. In these types of lines, the strip lengths of the hot rolled carbon steel are generally not welded or stitched together to form an endless strip, but are pushed or pulled through the line strip-by-strip. This allows small lots to be processed and provides some flexibility in production planning.
A typical push-pull pickling line PPPL for processing hot rolled carbon steel HRCS is shown in FIGS. 1 and 2. The processed sheet metal is uncoiled from an uncoiling reel 10 and directed through a processor 12 and a dividing shear 14. The sheet metal is then directed into a pickling tank 16 where the sheet metal is submerged in pickling solution to remove the scale. After leaving the pickling tank 16, the sheet metal is directed to a rinser 18, which removes the pickling acids from the sheet metal. The sheet metal is then dried in a dryer 20 before being directed to an inspection stand 22 and a dividing shear 24. A loop pit 26 may be provided to allow for changes in the speed of the line as the sheet metal passes through the various processing machinery. After exiting the loop pit 26, the sheet metal may pass through a steering pinch roll and a strip centering device 28. The sheet metal may then pass through a side trimmer 30, a braking stand 32, and an oiling machine 34 before being coiled on a tension reel 36.
For each production run, the hot rolled carbon steel strip HRCS may be uncoiled from the uncoiler 10 and threaded through each machine and station in the line to the coiling machinery and tension reel 36. In particular, the hot rolled carbon steel strip may be threaded through the pickling tank 16 containing a hydrochloric acid pickling solution so that the strip is immersed in the solution. Typically, the tanks 16 are formed from a granite material that not only withstands the reactivity of the hydrochloric acid pickling solution but also the wear induced by the leading edge of the steel strip during the process of threading the hot rolled carbon steel strip through the line during each coil's production run.
Hot rolled carbon steel HRCS may also be processed in a semi-continuous pickling line SCPL as shown in FIGS. 3-5. A semi-continuous pickling line SCPL includes some of the same stations and machinery as the push-pull processing line described above, but the semi-continuous line includes equipment and added loop pits that allow for differences in processing speeds at the front of the line and back of the line, and a welder or stitcher that enables the successive strips to be uncoiled and pulled through the line in a continuous fashion thereby avoiding the threading process in a push-pull processing line, which takes place for each coil's production run.
As shown in FIG. 3, the hot rolled carbon steel HRCS strip is uncoiled from an uncoiler 40 and directed through a processor 42 and a dividing shear 44. Then, a stitcher 46 joins the peeled-off, leading end of the coil to the trailing end of the then in-process strip. To allow for differences in the speed of processing between the strip being uncoiled and the then-in process strip, an entry loop pit 48 may be provided. Rollers 50 may also be provided around the stitcher 46 and/or the loop pit 48 to allow for needed variation in line speed during stitching. Once the strip exits the entry loop pit 48, the strip is directed through a pickling tank 52, a rinsing section 54, a dryer 56, and an inspection stand 58, as shown in FIG. 4. To allow for differences in processing speeds at the front of the line and back of the line, an exit loop pit 60 may be provided. As shown in FIG. 5, the strip exiting from the exit loop pit 60 may be directed to a steering pinch roll and strip centering device 62, a side trimmer 64, a braking stand 66, a dividing shear 68, and an oiling machine 70 before being coiled on a tension reel 72.
While hot rolled carbon steel HRCS may be processed in a push-pull pickling line PPPL or a semi-continuous pickling line SCPL depending upon the steel grades, the production quantity required, and the strip product dimensions, the processing of stainless steel strip involves different procedures and processes, which result in different processing line configurations, and traditionally exclude push-pull configured lines.
To descale stainless steel, an acid pickling solution is used that is more reactive than that used for hot rolled carbon steel. For instance, hydrofluoric acid is commonly used to pickle stainless steel. However, the use of hydrofluoric acid requires different design considerations for the processing line than lines using hydrochloric acid in hot rolled carbon steel processing. Hydrofluoric acid generally degrades granite, and thus the granite tanks that are traditionally used in lines processing hot rolled carbon steel must be replaced with other materials, for instance, plastic tanks. While plastic tanks are capable of withstanding the higher reactivity of the pickling solution used in lines processing stainless steel, such plastic tanks cannot withstand the wear induced from the leading edge of the strip during threading processes at the beginning of production runs. Thus, threading processes are often minimized or avoided in lines processing stainless steel to avoid premature reduction of the expected lifespan of the acid pickling tanks. Because threading processes are minimized in lines processing stainless steel, stainless steel is not traditionally processed in a push-pull processing line. Processing stainless steel in a push-pull processing line would require threading processes for each coil's production run, and the excessive threading processes would rapidly decrease the expected life of the plastic acid pickling tank. To avoid the issues associated with threading processes, stainless steel is traditionally processed in a semi-continuous or continuous processing line. The hydrofluoric acid traditionally used to pickle the stainless steel may be contained in plastic tanks, and because there are no repeated threading operations involved, there is diminished risk to damage to the plastic pickling tanks.
U.S. Pat. No. 7,601,226 describes a descaling apparatus that eliminates scale from the sheet metal and eliminates the pickling process that is used to remove scale from the surface of sheet metal. While U.S. Pat. No. 7,601,226 generally discloses a descaling apparatus that allows for descaling of both hot rolled carbon steel and stainless steels, U.S. Pat. No. 7,601,226 teaches such descaling by replacing the acid pickling tanks in a processing line with such a descaling apparatus. For instance, U.S. Pat. No. 7,601,226 and its child patents (including U.S. Pat. Nos. 8,062,095, 8,066,549, 8,074,331, and 8,128,460, the disclosures all of which are incorporated by reference herein) teach removing scale from hot rolled carbon steel and replacing the pickling tanks previously used in such processing lines with descaling apparatuses. Given the decades long practice of processing stainless steel in a semi-continuous or continuous processing line, U.S. Pat. No. 7,601,226 and its child patents merely suggest use of the disclosed descaling apparatus in a semi-continuous or continuous processing line, for instance, retrofitting a semi-continuous or continuous processing line with such a disclosed descaling apparatus. It was heretofore unappreciated that stainless steel may be processed in a push pull stainless steel processing line with such a descaling apparatus. The disclosure is directed to push-pull stainless steel processing line with a descaling apparatus, rather than the replacement of pickling tanks with a descaling apparatus in a conventional semi-continuous or continuous stainless steel processing line.